This invention relates to lock mechanisms having lock-operating spindles, and particularly to a frangible spindle that is configured to break at a predetermined location under an applied torque during an attack on the lock thereby inhibiting unauthorized access to the lock mechanism.
Conventional lock mechanisms, such as mortise locks, include a lock-operating spindle assembly having an inside spindle and an outside spindle rotatably coupled to the inside spindle. Typically, the inside spindle is coupled to a latch bolt-retracting inside hub and the outside spindle is coupled to a separate latch bolt-retracting outside hub. Generally, the inside hub is never locked so that the lock mechanism can always be operated by a user in the secured area during an emergency. The outside spindle generally carries a lever handle or the like.
A torque can be applied to the lever handle to fracture the outside spindle during an attack on the lock mechanism. Once the outside spindle is fractured and the lever handle separated from the lock mechanism, th attacker can apply an inwardly directed axial force to the exposed portion of the fractured outside spindle in an attempt to push the entire spindle assembly inwardly through the outside and inside hubs to expose the unlocked inside hub. Once exposed, the attacker can insert a tool into the unlocked inside hub and rotate or otherwise operate the hub by means of the tool to retract the latch bolt, thereby violating the security of the lock mechanism.
Heretofore, it has been known to provide an outside spindle having a cross-sectional area that is larger than the cross-sectional area of the inside spindle so that the an outside spindle defines an enlarged end face. The enlarged end face is formed on the end of the outside spindle that is rotatably connected to the comparatively smaller inside spindle and is larger than an inside spindle-receiving aperture formed in the inside hub. Although the comparatively smaller inside spindle will slide freely within the aperture formed in the inside hub to permit assembly, the enlarged end face of the outside spindle will engage the inside hub if the outside spindle is moved in a direction toward the inside hub during an attack. Thus, the enlarged end face blocks movement of a fractured outside spindle through the inside hub.
One problem with this known spindle assembly is that two different cross-sectional sizes of spindle stock must be provided and machined to make the inside spindle and the comparatively larger outside spindle. Further, the inside and outside latch bolt-retracting hubs must be formed with differently sized spindle-receiving apertures to accept the differently sized spindles. Installation can only be accomplished by inserting the inside spindle through the outside hub prior to engaging the inside spindle on the inside hub due to the enlarged cross-sectional area of the outside spindle. These requirements necessarily increase the costs and complicate the manufacture and assembly of known lock sets.
According to the present invention, a spindle assembly is provided for retracting a latch bolt of a lock set having separate independently rotatable inside and outside latch bolt-retracting hubs. The spindle assembly includes an inside spindle for operating the inside hub, an outside spindle for operating the outside hub, and means for rotatably interconnecting the inside and outside spindles. The outside spindle includes an inner spindle portion for engaging the outside hub, an outer spindle portion for supporting a lever handle or the like, and a frangible portion interconnecting the inner and outer portions.
The frangible portion is configured to transmit torque from the outer spindle portion to the inner spindle portion in response to application of a torque in excess of a first predetermined amount. This torque transmission twists the inner spindle portion to define means for blocking movement of the inner spindle portion through an axial spindle-receiving hole in the outside operating hub.
The frangible portion is also configured to break at a shear plane in response to application of a torque in excess of a second predetermined amount to the outer spindle portion of the outside spindle while the inner spindle portion is fixed against rotation. The second predetermined amount is greater than the first predetermined amount. The shear plane is located a predetermined distance away from the inside spindle.
The blocking means defined by twisting the inner spindle portion include a plurality of wedge members which are movable to block movement of the inner spindle portion through an axial spindle-receiving hole in the outside operating hub. The configuration of the frangible portion operates to define an effective size of the blocking wedges. The predetermined distance between the shear plane and the inside spindle is selected to maximize the effective size of each of the plurality of the blocking wedges.
In preferred embodiments of the present invention, the outside spindle is square or rectangular in cross-section and includes a plurality of stress risers such as notches which cooperate to define the shear plane. The cross-sectional area of the outside spindle at the shear plane is either smaller or weaker in comparison with the cross-sectional area of the rest of the outside spindle. In addition, the cross-sectional area at the shear plane must be large enough in comparison with the cross-sectional area of the rest of the outside spindle to transmit sufficient torque from the outer spindle portion to the inner spindle portion so that the inner spindle portion is twisted to define blocking means having at least a minimum effective size.
One feature of the present invention is the provision of a frangible portion configured to break at a shear plane located a predetermined distance away from the inside spindle. This feature advantageously defines the location at which the outside spindle will break under an applied torque, thereby ensuring that the portion of the fractured, twisted outside spindle remaining coupled to the outside hub following an attack will be of sufficient length to maximize the effective size of each of the twisted blocking wedges.
Another feature of the present invention is the provision of a frangible portion configured to undergo at least a predetermined amount of plastic deformation under an applied torque prior to fracture. This feature advantageously defines means for blocking movement of the inner spindle portion through the axial spindle-receiving hole in the inside and outside operating hubs thereby inhibiting access to the inside operating hub during an attack on the security of the lock.
Increasing the effective size of the twisted blocking wedges advantageously increases the "push-through" resistance of the outside spindle in relation to the outside and inside latch bolt-retracting hubs. Thus, an increase in the effective size of the twisted blocking wedges proportionately increases the threshold level of the amount of axially inwardly directed force that must be applied to the fractured outside spindle to push the outside spindle through the apertures in the outside inside hubs to expose the inside hub to attack. This feature advantageously reduces the likelihood that an attacker can gain access to the inside hub by fracturing and then applying force to the outside spindle.
Advantageously, the efficacy of the foregoing novel blocking feature is wholly independent of the cross-sectional size of the inside and outside spindles. Therefore, the inside and outside spindles of the present invention as well as the spindle-receiving apertures of both the inside and outside hubs can have the same cross-sectional area to avoid the shortcomings of known lock spindle assemblies.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.